1. Field of Invention
The present invention relates generally to structures for positioning one or more diagnostic or therapeutic elements within the body and, more particularly, to power control systems for use with the same.
2. Description of the Related Art
There are many instances where diagnostic and therapeutic elements must be inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation and atrial flutter which lead to an unpleasant, irregular heart beat, called arrhythmia.
Normal sinus rhythm of the heart begins with the sinoatrial node (or "SA node") generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular node (or "AV node"). This propagation causes the atria to contract in an organized way to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or "HIS" bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called "conduction blocks") can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called "reentry circuits," disrupt the normally uniform activation of the left and right atria.
Because of a loss of atrioventricular synchrony, the people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis.
Although pharmacological treatment is available for atrial fibrillation and flutter, the treatment is far from perfect. Many believe that the only way to treat the detrimental results of atrial fibrillation and flutter is to actively interrupt all of the potential pathways for atrial reentry circuits.
One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called "maze procedure" which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The incisions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The incisions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony. The incisions are also carefully placed to interrupt the conduction routes of the most common reentry circuits. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive. Thus, despite its considerable clinical success, only a few maze procedures are done each year.
More recently, maze-like procedures have been developed utilizing catheters and probes which can form lesions on the endocardium to effectively create a maze for electrical conduction in a predetermined path. Exemplary catheters are disclosed in commonly assigned U.S. Pat. No. 5,582,609. Exemplary surgical soft tissue coagulation probes employing a relatively shorter and stiffer shaft than a typical catheter are disclosed in commonly assigned U.S. patent application Ser. No. 08/949,117, filed Oct. 10, 1997, and U.S. patent application Ser. No. 09/072,835, filed May 5, 1998, both of which are incorporated by reference. Such probes may, for example, be used to treat atrial fibrillation in procedures wherein access to the heart is obtained by way of a thoracostomy, thoracotomy or median sternotomy.
Typically, the lesions are formed by ablating tissue with an electrode carried by the catheter or ablation probe. Electromagnetic radio frequency ("RF") energy applied by the electrode heats, and eventually kills (or "ablates") the tissue to form a lesion. During the ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), tissue coagulation occurs and it is the coagulation that kills the tissue. Thus, references to the ablation of soft tissue are necessarily references to soft tissue coagulation. "Tissue coagulation" is the process of denaturing proteins in tissue and heating the fluid within the tissue cell membranes which causes it to jell, thereby killing the tissue.
A primary goal of many soft tissue coagulation procedures is to create contiguous lesions (often long, curvilinear lesions) without over-heating tissue and causing coagulum and charring. Soft tissue coagulation occurs at 50.degree. C., while over-heating occurs at 100.degree. C. A problem in the related art is the issue of rapid turning on and off power when a coagulation electrode loses contact with tissue. Tissue in contact with a coagulation electrode acts as a load to the power circuit powering the electrode, usually an RF power circuit. When the coagulation electrode is pulled away from tissue or efficacious contact is lost, the load is removed, and the voltage output of the power circuit may change. Voltage may rise suddenly, which can cause problems when the electrode is reintroduced into contact with tissue, such as arcing or charring. As a safety consideration, the circuit in conventional systems is powered off for a predetermined period by turning off the power to the RF coagulation electrode when contact is lost.
However, the inventors herein have determined that powering a circuit completely off can result in a number of problems. For example, abrupt powering on of a coagulation electrode can char tissue if the voltage rise is too rapid. Additionally, powering a circuit completely off introduces the delay associated with powering the circuit back on into the procedure. Not only is this delay inconvenient, it can also be detrimental to the patient, especially since the loss of contact can happen many times during a procedure. For example, soft tissue coagulation probes can be used to perform a maze procedure during a mitral valve replacement, which requires cardiopulmonary bypass. The longer the patient is on bypass, the greater the likelihood of morbidity and mortality. Consequently, there is a need to quickly recover from a loss of electrode-tissue contact, without completely shutting off the power supply.
Another problem identified by the present inventors has been verifying that tissue is in contact with a coagulation electrode prior to or during a surgical or catheter-based procedure, which is generally termed electrode contact verification. This is a problem pervasive throughout all surgery being performed remotely, especially when direct visual line-of-sight is not present. The use of fluoroscopic techniques is somewhat inaccurate, and requires the use of human feedback. Accordingly, a need exists for an automated control system for electrode contact verification, and optionally with visual and/or audio feedback when there is loss of contact between the electrode and tissue.
Yet another problem identified by the present inventors is associated with tissue treatment efficacy when coagulating tissue. Specifically, because different tissues in the human body and between patients absorb energy at different rates, it is difficult to ascertain when proper tissue coagulation has been completed. Heretofore, ad hoc techniques have been used to determine when the soft tissue coagulation process has been completed. One technique is visual inspection. Another is applying coagulation energy for a predetermined period based on an estimate of the amount of time required to produce a therapeutic lesion. Such techniques are not always as reliable as desired. Thus, there is a need for accurately determining when tissue has been properly coagulated, so that coagulation may be automatically stopped.